1. Field of the Invention
The present invention relates generally to the field of optical waveguide fibers, and more particularly to methods of making low polarization-mode dispersion and low attenuation optical waveguide fibers.
2. Technical Background
A significant goal of the telecommunications industry is to transmit greater amounts of information, over longer distances, in shorter periods of time. Typically, as the number of systems users and frequency of system use increases, demand for system resources increases as well. One way of meeting this demand is by increasing the bandwidth of the medium used to carry the information. In optical telecommunication systems, the demand for optical waveguide fibers having increased bandwidth is particularly high.
In recent years, significant advancements have been made in the manufacture of optical waveguide fiber, which in turn have increased the usable light carrying capacity of the fiber. However, it is well known that electromagnetic radiation traveling through an optical waveguide fiber is subject to attenuation or lose due to several mechanisms. Although some of these mechanisms cannot be reduced, others have been eliminated, or at least substantially reduced.
A particularly problematic mode of optical fiber attenuation is attenuation due to absorption by the optical waveguide fiber due to impurities present in the light guiding region of the fiber. Particularly troublesome is the attenuation caused by the hydroxyl radical (OH), which can be formed in the optical waveguide fiber when a source of hydrogen is present in the fiber material, or when hydrogen available from several sources during the fiber manufacturing process diffuses into the glass.
Hydrogen bonds with the oxygen available in the SiO2 and/or GeO2 and/or other oxygen containing compounds in the glass matrix to form the OH and/or OH2 bonds. The attenuation increase due to OH or water in the glass can be as high as about 0.5 to 1.0 dB/km, with the attenuation peak generally accompanying the 1380 nm window. As used herein, the phrase, “1380 nm window” is defined as the range of wavelengths between about 1330 nm and about 1470 nm. The attenuation peak, generally referred to as the water peak, has prevented useable electromagnetic transmission in the 1380 nm window.
Until recently, telecommunication systems avoid the water peak residing in the 1380 nm window by operating in the 1310 nm window and/or the 1550 nm window, among others. With the advent of wavelength division multiplexing (“WDM”) and advancements in amplifier technology, which enable telecommunication systems to operate over broad wavelength ranges, it is likely that all wavelengths between about 1300 nm and about 1650 nm will be used for data transmission in optical telecommunication systems. Removing the water peak from optical waveguide fiber used with such systems is an important aspect of enabling system operation over this entire range.
In the manufacturing of an optical fiber, a variety of methods can be used to deposit the various soot layers. In the outside vapor deposition (“OVD”) process, the soot core blank is formed by depositing silica and germanium containing precursor constituents in the presence of oxygen onto a ceramic bait rod. As the bait rod is rotated, the precursor constituents are delivered to the flame burner to produce soot, and that soot is then deposited onto the bait rod. Once sufficient soot is deposited, the bait rod is removed, and the resultant soot core blank can be consolidated into a glass core blank. The soot core blank is typically consolidated by hanging the soot core blank in a consolidation furnace and heating the soot core blank to a temperature and for a time sufficient to consolidate the soot core blank into a glass. Preferably, prior to the consolidating step, the soot core blank is chemically dried, for example, by exposing the soot core blank to chlorine gas at an elevated temperature. The result is a cylindrical glass core blank having an axial hole along its centerline.
This glass core blank is then typically drawn, e.g., by positioning the glass core blank in a furnace, heating the core blank to a temperature of approximately 2000° C., and then redrawing or stretching the core blank into a smaller diameter core cane. During this redraw operation, the centerline hole of the core blank is collapsed by applying considerable vacuum (e.g., a pressure of less than 200 mTorr) along the centerline hole. These vacuum forces ensure complete closure of the glass core blank along the centerline. After the redraw step, the resulting core cane is then typically overclad with a layer of cladding soot by depositing a cladding soot, e.g. via an OVD deposition process. Once covered with sufficient cladding soot, the resultant soot overclad core cane is chemically dried and consolidated to form an optical fiber preform. While different processes (e.g. MCVD and others) may employ somewhat different processes to form components employed in the manufacture of preforms, many of them (e.g. MCVD) commonly end up with a cylindrical tube or other intermediate glass object having a hole therein, which is closed prior to drawing fiber therefrom. These manufacturing processes typically involve utilizing a vacuum at some point during the manufacturing process to close the hole or gap which is present between glass constituents without changing the outer diameter significantly.
The use of a vacuum to close the centerline and other holes in a glass core blank or other optical fiber preforms has some drawbacks. Such vacuum forces can result in a nonsymmetrical centerline profile of the cane, as shown, for example, in FIG. 1. FIG. 1 illustrates a cross section of core cane, indicated generally at 10, which includes a center point 12 surrounded by layers of glass 14. In FIG. 1, these glass layers 14 have an irregular, asymmetric shape, as a result of the application of the vacuum forces during redraw. Only at locations farther from the center point 12 do the layers of glass 16 begin to form more symmetrical and concentric circles or rings about the center point 12. The same non-symmetrical layers of glass present in the core cane will be present when that cane is eventually drawn into an optical fiber. Views of the centerline profile taken at different locations along the length of the core cane (or the optical fiber resulting therefrom) would also show core asymmetry. Further, the geometrical properties of the core cane and resultant optical fiber may change along the length thereof. More specifically, the specific asymmetrical shape at one location along the optical fiber might differ from the shape at another location along the optical fiber.
This asymmetric core geometry is believed to be a key cause of polarization mode dispersion (PMD), a form of dispersion which results when one component of light travels faster than another, orthogonal component. PMD is a severe detriment when present to any significant degree in single mode fibers, as it limits the data transmission rate of fiber-based telecommunications systems. More specifically, single mode fibers and multimode fibers both have an outside diameter of generally about 125 microns. However, single mode fibers have a small, e.g., about 8 micron, core diameter. This dimensional relationship makes single mode fibers extremely sensitive to polarization mode dispersion brought on by non-symmetric hole closure caused during fiber manufacture. Consequently, reduced PMD is a significant goal in fiber manufacture, especially in single mode fibers. In contrast to the small core size of single mode fibers, the core region of a multimode fiber commonly has a diameter of 62.5 microns or 50 microns. In multimode fibers, non-symmetric hole closure has resulted in the inability to tune refractive index profiles on the inner-most portion of the fiber adjacent the centerline. As a result, lasers used to launch light into such fibers are often offset some distance from the centerline of the multimode fiber to avoid this region of non-symmetric hole closure.
One method used to reduce PMD is spinning of the optical fiber during the fiber draw operation, wherein the fiber is mechanically twisted along its centerline axis while being drawn from the molten root of the blank. This twisting enables orthogonal components of light to couple to each other, thus averaging their dispersion and lowering PMD. However, spinning is a fairly complicated process for mitigating the effects of non-symmetric hole closure, and can impede the draw speed, cause coating geometry perturbations, reduce the strength of the optical fiber and so forth. It would therefore be desirable to manufacture fibers having a low PMD without resorting to such spinning techniques.
Additionally, asymmetric core geometry can cause variations in core diameter along the length of the fiber core so that transmitted light “sees” a different core cross-sectional area at different points along the length of the optical fiber. In addition, an asymmetric centerline profile can reduce the bandwidth of laser launched multimode fiber.
Another drawback to the use of vacuum forces to close the centerline hole is that such a process may result in voids along the centerline which further impair the transmissive properties of the optical fiber.
Despite the chemical drying and consolidating steps typically associated with the manufacture of optical waveguide fibers, such optical waveguide fibers have been found to exhibit a relatively high level of attenuation measured at approximately 1380 nm. Because telecommunication systems presently in use today do not operate at or in the immediate vicinity of 1380 nm, this shortcoming has been largely overlooked. With recent advancements made in WDM, amplifier technology, and laser sources, however, eliminating the water peak measured at 1380 nm has become a priority. The water peak is largely a result of water being trapped in the glass during the fiber manufacturing process. In the case of the OVD process, it is believed that a large portion of the water is trapped within the centerline region of the core cane prior to or during closure of the centerline hole. Although the blanks are chemically dried and sintered during consolidation, it has been found that the region of glass surrounding and defining the centerline hole is being rewet after drying. Most commonly, such rewetting occurs through the physisorption, chemisorption, or diffusion of water upon exposure of the centerline hole to an atmosphere that includes a hydrogen containing compound, such as, but not limited to water (H2O) following consolidation.